This study aims to establish what level of development has the Principle of Legality reached in national jurisdictions as well as in international law. The study analyses the developments and treatment of the Principle of Legality in international criminal law by means of an analysis of several national jurisdictions and international law; in Scotland, England, the United States, Mexico and South Africa. After this, the deduction of the rules of Legality were identified and compared with international developments in international human rights law and international criminal law was made and conclusions formulated. The various legal systems represent different ways of considering the Principle of Legality, taking into account the general aspects of retroactivity, strict construction and foreseeability as guidelines for this study. Therefore, it was not possible to extract a general understanding of the Principle of Legality, and even the consideration of minimum standards was a complex task. Despite these obstacles some conclusions could be reached regarding the evolution of the Principle of Legality in international criminal law. The Principle of Legality seems to be in a very early state of development in international criminal law. There seem to be some basic rules that are recognised in treaties, but the courts and the drafters of criminal definitions have not given the Principle the same scope at domestic legislatures and courts. However, the Rome Statute seems to provide for a system where the Principle of Legality could be respected, since its text gives the broadest scope of any treaty so far. However, for now there is still a lot of work to be done, if we aspire to create a respectful system of international criminal law, since the existing rules do not comply with the expected minimum standards.